Bacillary dysentery and enteric fevers continue to be important causes of morbidity in both developed and developing nations. Shigella cause an estimated >150 million cases of dysentery, and enteric fever occurs in >27 million people, annually (Bardhan, et al. (2010) Emerging Infec Diseases 16:1718-1723; Crump, et al. (2004) Bulletin of the WHO 82:346-353; Crump and Mintz (2010) Clin Infec Diseases 50:241-246; WHO (2005) Guidelines for the control of shigellosis . . . ). Shigellosis and enteric fevers together cause >250,000 deaths annually (Bardhan, et al. (2010) Emerging Infec Diseases 16:1718-1723; Crump, et al. (2004) Bulletin of the WHO 82:346-353), demonstrating a continuing need for a multi-valent vaccine for protection against these diseases. Importantly, two thirds of all Shigella cases occur in children under the age of five years (Kotloff et al. (1999) Bull. WHO, 77:651-666; Kweon (2008) Curr. Opin. Infect. Dis., 21:313-318), and enteric fevers are most common in young school-age children (Crump and Mintz (2010) Clin Infec Diseases 50:241-246).
The genus Shigella includes four species; S. dysenteriae, S. flexneri, S. boydii and S. sonnei, also designated as serogroups A, B, C and D, respectively. The first three species, respectively, are further divided into serotypes based upon differences in LPS structures. Upon ingestion of contaminated food or water, Shigella cause an acute invasive infection of the large intestine that typically results in severe abdominal cramps, fever, and dysentery (i.e., small volume <5 ml stools comprised of mucus, polymorphonuclear neutrophils, necrotic tissue, and streaks of blood). S. boydii and S. sonnei oftentimes cause a milder disease when compared to S. dysenteriae and S. flexneri. S. flexneri is responsible for most endemic infections in developing countries. S. sonnei is the species responsible for most endemic infections observed in industrialized countries. The US CDC estimates an incidence of ˜450,000 cases of S. sonnei disease in the US each year, which occurs mostly in child daycare facilities. S. sonnei is also responsible for a considerable amount of morbidity in developing countries such as Thailand, where it is the cause of ˜95% of shigellosis (Mead et al. (1999) Emerg. Infect. Dis., 5:607-625; Putthasri et al. (2009) Emerg. Infect. Dis., 15:423-432). S. dysenteriae serotype 1 (Sd1) is especially important, as it causes severe dysentery plus hemolytic uremic syndrome (as a result of producing the potent cytotoxin Shiga toxin), typically resulting in a higher mortality rate than infections due to other Shigella species. Furthermore, Sd1 classically causes large epidemics with high attack rates (World Health Organization (2005) Guidelines for the Control of Shigellosis, Including Epidemics Due to Shigella dysenteriae Type 1, ISBN 924159330X).
Currently, there is no licensed vaccine available to prevent the occurrence of shigellosis. Increasing multiple antibiotic resistance in Shigella commonly thwarts local therapies. As a result, the World Health Organization considers development of a vaccine against shigellosis a top priority. Most importantly, there is a global public health need for a vaccine to prevent shigellosis in endemic populations, travelers, and the military. Due to the existence of a large number of Shigella serotypes (>40), some investigators have attempted to find surface antigens common to most serotypes (Kaminski et al. (2009) Expert Rev. Vaccines, 8:1693-1704). Despite significant efforts, common surface proteins by themselves (e.g., ipaA,B,C,D) do not appear to stimulate significant or sustained protective immunity to Shigella infection. However, there is considerable evidence that protective immunity is directed primarily against Shigella serotype specific LPS O-antigen, which highlights the importance of O-antigens as targets for vaccine development (Ferreccio et al. (1991) Am. J. Epidemiol., 134:614-627; DuPont et al. (1972) J. Infect. Dis., 125:5-11; DuPont et al. (1972) J. Infect. Dis., 125:12-16).
Indeed, lipopolysaccharide (LPS) alone has been shown to be a potent vaccine antigen for specific protection against shigellosis. The plasmid cloning of heterologous LPS biosynthetic genes and the expression in Ty21a of either S. sonnei or of S. dysenteriae 1 LPS's have previously been reported. The resulting plasmids encoding Shigella LPSs were reasonably stable for more than 50 generations of growth in non-selective media, but they still contained an objectionable antibiotic resistance marker. The deletion of this antibiotic resistance marker resulted in significant plasmid instability.
Based upon specific Shigella serotype prevalence worldwide and previous studies of serotype cross-protection among Shigellae, Noriega, et al. (1999 Infection and Immunity 67:782-788) have suggested that a multivalent vaccine containing LPSs of S. Sonnei, S. dysenteriae 1, S. flexneri 2a, S. flexneri 3a, and S. flexneri 6 could protect against˜85% of shigellosis worldwide.
The live, attenuated, oral vaccine Salmonella enterica serovar Typhi strain Ty21a has been utilized extensively as a broad-based oral vaccine vector for the expression of various foreign antigens (Xu et al. (2007) Vaccine, 25:6167-6175; Xu et al. (2002) Infect. Immun., 70:4414-4423; Osorio et al. (2009) Infect. Immun., 44:1475-1482). Ty21a is the only licensed, live, attenuated vaccine for protection against typhoid fever. Moreover, it has been safely administered to more than 200 million recipients around the world. As a whole-cell vaccine, Ty21a induces mucosal, humoral, and cellular immunity, leading to high-level, long-term protection (i.e., virtually undiminished protection at the end of 7 full years) against typhoid fever (Levine, et al. (1999) Vaccine 17 Suppl 2: S22-27) with considerable evidence of cross-protection against both S. Paratyphi A and B (Bardhan, et al. (2010) Emerging Infec Diseases 16:1718-1723; D'Amelio et al. (1988) Infect. Immun., 56:2731-2735; Levine, et al. (2007) Clin Infec Diseases 45 Supp 1:S24-28; Schwartz, et al. (1990) Archives Internal Med 150:349-351; Wahid, et al. (2012) Clin and Vaccine Immunol CVI 19:825-834).